The present invention relates to an improved gate design for injection molding of rubber compounds. More particularly, the present invention relates to an improved gate design for injection molding of rubber compounds with increased gate heating efficiency and reduced cycle time for curing the rubber part being injection molded.
In a typical rubber injection molding process, the uncured, viscous rubber compound is introduced into the elongated barrel of an injection molding machine at ambient temperatures. It is advanced through the barrel towards a mold connected to the downstream end of the barrel, usually by either a rotating screw conveyor or a reciprocating ram or piston disposed in the barrel. As the rubber compound advances, it is heated by heat conduction and mechanical shear heating in the barrel to reduce its viscosity and render it more flowable and amenable to subsequent injection into the mold. Typically, the less viscous the rubber compound, the more easily it flows through the runners and gates and the more easily it fills the mold cavity to produce a satisfactorily molded object.
Since curing of the rubber compound is a xe2x80x9ctime at temperaturexe2x80x9d phenomenon, the heating also serves to supply some of the xe2x80x9ctime at temperaturexe2x80x9d requirement in the barrel without prematurely curing or scorching the compound in the barrel. This increase in temperature also reduces the xe2x80x9ctime at temperaturexe2x80x9d required in the mold and consequently the vulcanization cycle time. As known in the art, most rubber compounds can be cured through either a shorter exposure to a higher temperature or a longer exposure to a lower temperature, and it is this phenomenon that is referred to herein by the term xe2x80x9ctime at temperature xe2x80x9d.
Cure time in ram injection rubber molding, for example, consists of three separate and distinct xe2x80x9ctime at temperature xe2x80x9d periods. The first is the xe2x80x9ctime at temperaturexe2x80x9d during the compounding and storage of the material prior to introducing the rubber into the upstream end of the barrel of the injection molding apparatus and is referred to as the xe2x80x9cprocess scorch time xe2x80x9d. The second xe2x80x9ctime at temperaturexe2x80x9d is the xe2x80x9cresidual scorch timexe2x80x9d or the time permitted in the barrel of the injection molding apparatus before incipient cure occurs. The higher the xe2x80x9ctime at temperaturexe2x80x9d during the process scorch time, the shorter will be the xe2x80x9ctime at temperaturexe2x80x9d permitted for the residual scorch time in the barrel of the apparatus. The third xe2x80x9ctime at temperaturexe2x80x9d is the xe2x80x9cvulcanization timexe2x80x9d of the compound within the mold itself. The three periods of time together comprise the total cycle time and the nature and degree of the xe2x80x9ctime at temperaturexe2x80x9d of the first two periods have an effect upon the third period, i.e., the cure or vulcanization time. Thus, rubber compounds with the same formulation at higher temperatures will vulcanize more quickly than the same rubber compounds at lower temperatures. In most injection molding operations, a smaller portion of the xe2x80x9ctime at temperaturexe2x80x9d requirement is supplied in the barrel of the injection molding apparatus, i.e., the residual scorch time, and larger portion of the xe2x80x9ctime at temperaturexe2x80x9d requirement is supplied in the heated mold, i.e., the vulcanization time.
In addition to the cumulative effect of the above xe2x80x9ctime at temperaturexe2x80x9d periods, there is a critical temperature range for each rubber compound called the xe2x80x9ccritical residual scorch temperature rangexe2x80x9d at which the vulcanizing of rubber is initiated. These temperature ranges are known to or can be determined by those skilled in the art. For typical rubber compounds the critical scorch temperature range can be between about 240xc2x0 F. (115xc2x0 C.) and 320xc2x0 F. (160xc2x0 C.). Just above that temperature range the compound will begin to xe2x80x9cscorchxe2x80x9d or vulcanize prematurely in some period of time, which may be minutes or seconds. Below that temperature range, vulcanization may require hours.
In a typical rubber injection molding process, the objective is to heat the rubber compound to the maximum temperature, just below the critical scorch temperature range, which will produce the lowest viscosity of the compound at this limited temperature. The inability to supply more temperature or heat energy or xe2x80x9ctime at temperaturexe2x80x9d in the barrel so that vulcanization time in the mold can be reduced has been a continuous problem in prior art processes and apparatus. It is toward this problem that the present invention is generally directed.
The rubber compound is usually initially heated by externally heating the barrel of the apparatus electrically, with a steam jacket or from some other such external heat source and transferring the heat by conduction from the hot barrel wall into the mass of the rubber compound moving downstream through the barrel. Additional heat is supplied to the compound by frictional forces and by shearing of the rubber compound which occurs in the barrel and screw, the sprue, and the runners and gate of the mold. In many cases, this additional heat is an important factor upon which the vulcanization depends. Once the compound is in the mold cavity, additional heat is supplied to the compound and the compound is held in the mold for the required xe2x80x9ctime at temperaturexe2x80x9d to vulcanize and complete the cure.
Vulcanizing cycle time can be reduced if the compound can be rapidly and uniformly heated to a higher temperature and then quickly injected into the mold so that more of the xe2x80x9ctime at temperaturexe2x80x9d required to cure the rubber compound occurs when the compound enters the mold. However, the rubber compound cannot be exposed to high temperatures for even short periods of time in the barrel or undesirable curing or scorching would take place before the rubber compound enters the mold. One difficulty encountered in attempting to quickly and uniformly heat the rubber compound, while it is still in the barrel, stems from the poor thermal conductivity of the rubber compound. This makes it difficult to use external heat to quickly heat the rubber compound to a uniform temperature throughout. To rapidly obtain the desired temperature in the portions of the rubber compound distant from the heat source, e.g., the electricity or steam heated barrel wall, it is necessary for the heat source to have a temperature substantially above that desired in the rubber compound. This produces local hot spots in the rubber compound in proximity to the barrel wall which cause formation of an undesirable skin of scorched rubber compound or prematurely vulcanized rubber compound near the barrel wall. This can produce undesirable pieces of cured rubber compound in the material before it even reaches the mold for final curing of the rest of the product. These pieces of cured rubber compound can clog the sprue and mold runners and ruin the molded product. As a result, the temperature of the barrel wall is usually maintained sufficiently low to avoid such hot spots and is kept below the critical scorch temperature. Consequently, the compound temperature does not become excessively high so that only a relatively small portion of the xe2x80x9ctime at temperature xe2x80x9d required to cure the rubber compound is provided in the barrel. Furthermore, the temperature of the compound varies throughout, with the compound more distant from the barrel wall cooler than that close to the wall. The result of these factors is that a longer vulcanization cycle is required once the rubber compound is injected into the mold in order to provide the xe2x80x9ctime at temperaturexe2x80x9d required to complete the cure of the entire mass of rubber material.
Various techniques have been proposed to more quickly and uniformly heat the compound entering the mold by, for example, heating the rubber compound to high temperatures in the barrel. The heating of the compound increases its temperature and reduces its viscosity to produce a heated, plasticized, more flowable material suitable for injection into the mold. However, since the temperature of the injected material is not uniform, and since the xe2x80x9ctime at temperaturexe2x80x9d in the barrel is well below that required to cure the compound, relatively long vulcanizing cycles in the mold are still required.
The gate design is another important variable in the process for minimizing mold cycle time while still producing high quality molded parts. The gate is typically designed to rapidly heat rubber from about 240xc2x0 F. (115xc2x0 C.) to about 320xc2x0 F. (160xc2x0 C.) through a combination of both shear and conduction heating. The shear heating is generated by the rubber compound in the gate being forced to flow into a smaller area than that of the barrel from which the rubber compound is directed into the gate and the conduction heating is from the heat being conducted back into the rubber compound in the gate.
In an effort to reduce molding cycle times, faster injection rates are needed to minimize injection times. In some prior art molding applications, a continuous, thin flat cross-sectioned, film-type gate design is used, as shown in FIGS. 1 and 2. This type of gate design is good, but its rubber heating efficiency is reduced with increased injection speed because the shear heat generated by the faster injection speed is primarily absorbed in the gate wall or by the mold itself.
Based on experimental data, an excessive amount of heat is generated in the flat gate design by shear heating, which increases with injection speed. The heat from shear heating is usually generated within an outer 10% layer of the rubber flow thickness. Since rubber is a poor conductor of heat, this heat does not conduct quickly enough to evenly heat all of the rubber flowing through the gate. Instead, much of the shear heat from the rubber flowing through the gate is transferred into the wall of the metal gate itself or into the mold. Moreover, overheating of the gate can cause undesirable surface sink marks and rubber scorch.
There is a definite need to overcome the prior art problems which limit the decrease in the total cycle time for molding and vulcanizing parts of rubber in a mold.
It is an object of the present invention to provide an improved gate for directing rubber into an injection molding device being as defined in one or more of the appended claims and, as such, having the capability of being constructed to accomplish one or more of the following subsidiary objects.
It is an object of the present invention to provide an improved gate for directing rubber into a mold cavity of an is injection molding device and a method of thermally mixing streams of rubber flowing through the gate to reduce the temperature variation of the rubber flowing into the mold cavity.
It is a further object of the present invention to provide an improved lattice gate for directing streams of rubber into a mold cavity of an injection molding device wherein the cycle time is decreased as compared to a flat gate having a comparable flow rate.
It is still a further object of the present invention to provide an improved lattice gate for directing rubber into a mold cavity so that the minimum cure time is reduced.
In accordance with an embodiment of the invention, a lattice gate system for directing rubber into a mold cavity of an injection molding device includes a sprue for directing rubber into the gate; an elongated distribution channel connected to the sprue for receiving the rubber; and a plurality of separate, intersecting flow channels connected at one end to the elongated distribution channel and at the opposite end to the inlet of a mold for directing streams of rubber into the mold cavity.
Further, in accordance with the invention, the separate intersecting flow channels of the gate intersect at an angle of about 60xc2x0 to about 140xc2x0 and preferably about 90xc2x0 to about 120xc2x0 with respect to each other.
Also in accordance with the invention, the gate includes: a lattice gate sprue plate having a flat inner surface with an inlet end and an outlet end, and a plurality of separate flow channels extending parallel to each other and disposed at an angle of about 30xc2x0 to about 70xc2x0 with respect to a centerline extending through the lattice gate sprue plate from the inlet end to the outlet end; a lattice gate plate having a flat inner surface with an inlet end and an outlet end, and a plurality of separate flow channels extending parallel to each other at an angle of about 30xc2x0 to about 70xc2x0 with respect to a centerline extending through the lattice gate plate from the inlet end to the outlet end; and the flat inner surface of the lattice gate sprue plate being abutted against the flat inner surface of the lattice gate plate. The separate flow channels in the lattice gate plate intersect the separate flow channels in the lattice gate sprue plate whereby the rubber streams flowing through the separate flow channels in the lattice gate plate and the lattice gate sprue plate mix both physically and thermally and generate more shear heating at the intersections of the separate flow channels.
Moreover, in accordance with the invention, the lattice gate sprue plate has a sprue inlet bore extending through the lattice gate plate near the inlet end; and the lattice gate plate has a sprue inlet counterbore extending into the lattice gate plate near the inlet end and an elongated distribution channel extending across the lattice gate plate and connected to the sprue inlet counterbore and to the plurality of separate flow channels extending through the lattice gate plate and the lattice gate sprue plate from the elongated distribution channel to the outlet ends.
In accordance with an embodiment of the invention, there is provided a process for injecting a stream of rubber into an injection molding device comprising the steps of: directing the stream of rubber from the sprue channel into an elongated distribution channel; directing the a stream of rubber from the elongated distribution channel into a plurality of separate, intersecting flow channels connected at one end to the elongated distribution channel; thermally mixing rubber flowing through the separate flow channels at the intersection of the separate flow channels; and directing the rubber exiting the intersecting flow channels into a mold.
Further in accordance with the invention, the process includes the step of directing the rubber into a plurality of separate, intersecting flow channels wherein the separate intersecting flow channels intersect so that the flat surface of the flow channels contact each other.
In accordance with a second embodiment of the invention, the lattice gate can have a ring design to inject rubber into a circular, annulus-shaped mold having intersecting flow channels.